This invention is directed at an isotactic stereoblock poly(lactic acid) and to methods of making an isotactic stereoblock poly(lactic acid).
Poly(lactic acid)s, PLAs, are considered to have utility for medical, agricultural, and packaging application due to their biocompatibility and biodegradability. In view of this, it is desirable to provide new stereospecific forms of poly(lactic acid)s and new methods of preparing known forms of poly(lactic acid)s.
A convenient synthetic route to PLAs is the ring-opening polymerization of lactide, the cyclic diester of lactic acid. A range of metal alkoxide initiators have been reported to polymerize lactide with retention of configuration. For example, these initiators have been reported to polymerize optically active (R,R)-lactide or (S,S)-lactide to produce isotactic poly(lactic acid). Moreover, these initiators have been reported to polymerize rac-lactide to produce amorphous, atactic polymers.
Heterotactic poly(lactic acid) is a stereospecific polymer that has alternating pairs of stereogenic centers in the main chain. This poly(lactic acid) and the method of making it are disclosed in U.S. patent application Ser. No. 09/707,980, filed Nov. 8, 2000 now U.S. Pat. No. 6,316,590.
Spassky, N., et al., Macromol. Chem. Phys. 197, 2627-2637 (1996) reported the kinetic resolution of racemic lactide (rac-lactide) with the methoxide variant of the R-enantiomer of catalyst used in the invention herein. The high melting material that formed is considered to have a tapered stereoblock microstructure, i.e., there was not a sharp distinction between blocks and there were effectively two blocks.
It has been discovered herein that by polymerizing rac-lactide in the presence of a racemic version of the catalyst used by Spassky, et al., that isotactic stereoblock poly(lactic acid) having several total blocks where each block contains several monomer units, is formed.
One embodiment herein, denoted the first embodiment, is directed to isotactic stereoblock poly(lactic acid) having a number average molecular weight ranging from 10,000 to 200,000 grams per mole, with, on average, an equal number of poly (R) and poly (S) blocks where each block contains an average of 5 to 50 monomer units.
Another embodiment herein, denoted the second embodiment herein, is directed to preparing isotactic stereoblock poly(lactic acid) of the first embodiment herein comprising polymerizing rac-lactide in the presence of a racemic catalyst consisting of: 
and the corresponding S-enantiomer, where R is C1-C4 alkyl which is straight chain or branched.
Still another embodiment herein, denoted the third embodiment herein, is directed to preparing isotactic stereoblock poly(lactic acid) of the first embodiment herein comprising polymerizing rac-lactide in the presence of a racemic catalyst consisting of: 
and the corresponding S-enantiomer, where R is C1-C4 alkyl which is straight chain or branched.
The number average molecular weights (Mn) herein are determined by gel permeation chromatography (GPO).
We turn now to the embodiment directed to isotactic stereoblock poly(lactic acid) having a number average molecular weight ranging from 10,000 to 200,000 grams per mole, with, on average, an equal number of poly (R) and poly (S) blocks where each block contains an average of 5 to 50 monomer units.
The stereoblock poly(lactic acid) prepared in Example I herein has a number average molecular weight of 22,600 grams per mole, with, on average, an equal number of poly (R) and poly (S) blocks where each block contains an average of 11 monomer units. The stereoblock (poly(lactic acid) prepared in Example II herein has a number average molecular weight of 29,560 grams per mole, with, on average, an equal number of poly (R) and poly (S) blocks where each block contains an average of 10 monomer units.
A structural formula for the isotactic stereoblock poly(lactic acid) of the first embodiment is: 
where n averages from 5 to 50 and m averages from 2 to 200. In the formula I, n is the average number of monomer units in a block, and m is the number of blocks.
We turn now to the second embodiment herein, that is the embodiment directed to preparing isotactic stereoblock poly(lactic) acid comprising polymerizing rac-lactide in the presence of a racemic catalyst consisting of: 
and the corresponding S-enantiomer, where R is C1-C4 alkyl which is straight chain or branched. In the catalyst, the R- and S-enantiomers are in a 1:1 ratio.
The rac-lactide, that is racemic lactide, is an admixture of (R,R)-lactide and (S,S)-lactide in a 1:1 ratio. It is commercially available. (R,R)-lactide has the formula: 
The catalyst (II) is prepared, for example, by synthesizing the R-enantiomer and the S-enantiomer and forming or using an admixture thereof in a 1:1 ratio. The R-enantiomer and the S-enantiomer of the ligand of the complex can be prepared according to the procedure described in Bermardo, K. D., et al., New J. Chem. 19, 129-131 (1995). Complex (II) and the corresponding S-enantiomer can be formed by heating solution of enantiomeric ligand and the appropriate aluminum alkoxide in toluene.
Examples of the catalyst are admixtures of corresponding R and S enantiomers of complex and include admixtures of complexes with the formula (II) where R is methyl or where R is isopropyl and the corresponding S-enantiomers.
For the polymerization, the mole ratio of monomer to aluminum can range, for example, from 10:1 to 1,000:1.
The polymerization is carried out, e.g., in an aprotic solvent, e.g., toluene or benzene, at a temperature ranging 50xc2x0 C. to 100xc2x0 C., e.g., 70xc2x0 C.
The stereoblock poly(lactic acid) of the Example I is prepared by the method of the second embodiment.
Other stereoblock poly(lactic acid)s herein are prepared using other racemic aluminum alkoxide catalysts in place of catalyst (II).
We turn now to the third embodiment herein, that is the embodiment directed to preparing isotactic stereoblock poly(lactic) acid comprising polymerizing rac-lactide in the presence of a racemic catalyst consisting of: 
and the corresponding S-enantiomer, where R is C1-C4 alkyl which is straight chain or branched. In the catalyst, the R- and S-enantiomers are in a 1:1 ratio.
The method of the third embodiment is the same as the method of the second embodiment except for the catalyst.
The catalyst for the third embodiment can be prepared as follows. The ligand used to prepare the catalyst can be prepared according to the procedure described in Kanoh, S., et al., Polymer Journal 19, 1047-1065 (1987). The racemic catalyst is prepared, for example, by heating a solution of racemic ligand and the appropriate aluminum alkoxide in toluene.
Elements of the invention are described in a publication of Ovitt, J. M., et al., titled xe2x80x9cStereoselective Ring-Opening Polymerization of rac-Lactide with a Single-Site, Racemic Aluminum Alkoxide Catalyst: Synthesis of Stereoblock Poly(lactic acid),xe2x80x9d Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 4686-4692 (2000), which is incorporated herein by reference.
The invention is illustrated by the following working examples: